Adjust x axis height when tick labels are rotated using a negative value

[GDFaber]

Description

The x axis labels can be rotated using the rotate setting. If no axis height was given explicitly, it is calculated in a way that fits the rotated texts. This works for positive values, but if you set rotate: -45 for example, the height is not adjusted properly.

Steps to check or reproduce

1. On the billboard.js homepage, visit the RotateXAxisTickText example.
2. Modify the rotate setting: rotate: -45.
3. Remove the height setting (and the trailing comma in the line before).

[GDFaber]

I think this can be solved by modifying getHorizontalAxisHeight(id) to use the absolute rotation value for its calculations in case of x axis. I'll add a PR later.